The present invention relates to a process for preparing 4,4'-difluorobiphenyl, and more particularly, to an improved such process wherein a high yield of 4,4'-difluorobiphenyl is realized. 2. Description of the Prior Art.
Because of their outstanding chemical, heat and electrical insulation resistance as well as their superior rigidity when compared to other thermoplastics, poly(arylene sulfide) resins have found favor for forming reinforced plastic composites.
A poly(arylene sulfide) resin which has been found to have a very high melting point and to be particularly suitable for use at high temperatures is comprised of biphenylene sulfide polymer. Poly(biphenylene sulfide) resin retains its hardness at temperatures as high as about 700.degree. F. and, in addition, has a high retention of mechanical properties at elevated temperatures.
Heretofore, poly(biphenylene sulfide) resin has been prepared from sodium sulfide and 4,4'-dichlorobiphenyl. Recently, however, as a result of 4,4'-dichlorobiphenyl having been found to be hazardous to the environment and to human health, poly(biphenylene sulfide) resin has been prepared with sodium sulfide and 4,4'-difluorobiphenyl.
4,4'-difluorobiphenyl has heretofore been prepared by a process utilizing a Grignard coupling reaction. In accordance with the prior process, p-halofluorobenzene is reacted with magnesium in a solvent to form halomagnesiumfluorobenzene, the Grignard reagent. The Grignard reagent is then reacted with additional p-halofluorobenzene (the coupling reagent) in the presence of a catalyst comprised of palladium chloride or palladium supported on charcoal. This prior process results in a yield of about 80% 4-4'-difluorobiphenyl when the Grignard reagent is formed from pbromofluorobenzene and the coupling reagent is p-iodofluorobenzene. In addition, about 8% of a difluoroterphenyl byproduct is formed in the reaction. Thus, the heretofore utilized process produces a relatively low yield of 4,4'-difluorobiphenyl. In addition, the process is disadvantageous in that p-iodofluorobenzene is expensive, and a high molar ratio of the palladium chloride or palladium/charcoal catalyst is required. While p-bromofluorobenzene is much less expensive than p-iodofluorobenzene, when p-bromofluorobenzene is substituted for p-iodofluorobenzene in the coupling reaction, a very low yield of 4,4'-difluorobiphenyl is obtained.
Thus, there is a need for an improved process for preparing 4,4'-difluorobiphenyl which produces a high yield, which requires a lower quantity of catalyst and which, if desired, can utilize only less expensive p-bromofluorobenzene starting materials.